The term cellulitis is a medical term describing a bacterial infection of the skin and tissues beneath the skin. Although it is usually easily treated with antibiotics given either orally or through a vein (intravenously), knowing which route of antibiotic treatment to prescribe to a person attending an Emergency Department with cellulitis is not clear.

A Clinical Prediction rule (CPR) is a decision-making tool that comes from original research as opposed to the opinion of experts. We intend to create a preliminary CPR to decide which patients require oral and which patients require intravenous antibiotics for cellulitis from their first visit to an emergency department. The aim of this is to provide safer care by reducing the risk of a patient returning to the hospital with a worsening infection. It will also promote more cost-effective care by reducing hospital re-attendance rates and wasted antibiotics.

Patients attending the department with cellulitis who are suitable for oral antibiotic treatment will be enrolled into this study. A separate doctor will re-examine at least 10% of study participants in order to reduce bias. A set of physical signs and symptoms will be recorded from each patient in order to determine which ones are associated with them "failing" prescribed oral treatment. A study investigator will then phone the patient after 14 days to see whether they are better or whether they required intravenous antibiotics to get better.

Inclusion Criteria:

• Age >16 years
• Suitable for treatment with flucloxacillin 500mg -1gram qds monotherapy or a suitable alternative for penicillin allergic patients as listed in the local prescribing guidelines.
• Appearance of typical, plaque-like area of erythema over any body part excluding the perineum within the preceding 5 days with any 2 of the following signs:
• 1.Increased warmth over affected area
• 2.Swelling of affected area
• 3.Pain over affected area
• 4.Regional lymphadenopathy

Exclusion Criteria:

• Requirement for IV antibiotics as decided by the treating clinician.
• Age less than 16 years.
• No telephone or access to a telephone.
• Abscess alone without co-existing signs of cellulitis
• Mammalian bite wounds.
• Infected diabetic foot ulcer
• Necrotising soft tissue infections.
• Perineal cellulitis.
• Suspected septic arthritis or osteomyelitis.
• Decubitus ulcers.
• Bilateral cellulitis (as this entity rarely exists).
• Acute lipodermatosclerosis.
• Acute dermatitis.
• Venous stasis dermatitis.
• Deep vein thrombosis.
• Pregnancy.
• Cognitive impairment.
• Any patient who through language barrier or diminished capacity is unable to understand the scope of the study

Adult patients aged >16 years attending the ED with cellulitis as the primary diagnosis will be consecutively recruited. Only patients who are deemed suitable for an oral antibiotic will be enrolled. Patients who require IV therapy will not be enrolled. Cellulitis may arise de novo, or from a recognised cause such as a wound or ulcer. In order to generate an externally valid clinical prediction rule, we will include all patients attending the ED with cellulitis, including those who may have already been commenced on oral antibiotics (for example, by their general practitioner).